Human powered vehicles have long been used as a means of transportation and recreation. However, virtually all human powered vehicles known in the prior art have involved a relatively rigid frame, which transmits directly to a rider the shocks resulting from potholes, rocks and other unevenness in the riding surface. This has resulted in discomfort to the rider which is at least undesirable, if not unacceptable.
The typical solution to this problem found in the prior art has been to include a shock absorbing suspension on the vehicle, similar to those found on motorcycles or on cars. One of the difficulties with nearly all prior art designs is that they absorb a significant amount of driving force which would otherwise provide forward movement. While this loss is not significant for a motorized vehicle, it is undesirable where the only motive force is the muscle power of a human being.
The energy loss referred to results from the fact that in virtually all prior art suspension devices, there is no rigid connective member between the input of the driving force and the driving wheel. As a result of this, the spring or elastic member allowing suspension movement is affected by the input force. This will typically be perceived as the vehicle settling or rising in response to the input force.
The majority of suspension systems for human powered vehicles in the prior art suffer from chain tension induced compression of the suspension. One of the earliest and simplest examples of this type is illustrated in U.S. Pat. No. 564,319 to Travis. In this version, the top run of the chain in tension pulls against the rear wheel axle and the pivot is located below this portion of the chain. The result is that the swing arm is pulled in a direction that compresses the suspension. This action can be identified by the rider as a significant settling or bobbing of the vehicle in response to peddling input. This action is undesirable because a significant portion of the energy normally routed directly to driving the rear wheel is absorbed by the action of compressing the suspension.
More recent attempts to solve this problem of wasted energy have raised the pivot to a point above the top run of the chain as shown in FIG. 1. These versions suffer from chain tension induced extension of the suspension. The prior art in FIG. 1 shows that when the tensioned top run 21 of chain 23 pulls against rear sprocket 43 and pivot 30 is above chain 23, then chain 23 becomes a means to pull swing arm 40 in the direction of tension force arrow T so as to extend spring suspension 200. This action can be identified by the rider as a significant rising or bobbing of the vehicle relative to ground G in response to pedaling input. This bobbing action is undesirable because a significant portion of the energy ideally routed directly to driving the rear wheel 35 is absorbed by the action of extending the suspension 200.
A further example of the prior art is U.S. Pat. No. 4,789,174 to Lawill, which illustrates a bicycle having a suspension system that is a further attempt to circumvent the problems of chain tension induced actuation thereof. This version comprises an elaborate multi-pivoted interconnection of swing arms, control arms and hub plates configured in a trapezoidal arrangement and connected to a bicycle seat riser tube. If one skilled in the art studies this system closely, it becomes apparent that this system does not completely eliminate chain tension induced actuation of the suspension, and instead adds unnecessary complexity to the suspension system, thereby dictating structural, maintenance, manufacturing and weight compromises.
Another version known in the art is illustrated in U.S. Pat. No. 4,669,747 to Groendal. While this design may overcome the problem of chain tension induced actuation, it has other flaws which render it unacceptable. For example, this bicycle frame provides almost no suspension to the seat. The shock from a bump in the riding surface travels directly up from the rear axle and through the frame to the seat. Further problems with this design are apparent upon application of the front brake. A hard braking force at the front wheel causes the wheelbase of the frame to shorten, the cable goes slack, and a completely unacceptable "wind-up" of the top tube occurs.
This braking problem is only partially overcome in the more modern version shown in U.S. Pat. No. 5,080,284 to Groendal, et al. This design suffers from an unacceptable shortening and lengthening of the rider position. As the suspension reacts to a bump in the riding surface, the handlebars move closer to the saddle, thus compromising the ideal riding position of the rider.
While there are some other versions of suspension systems which do not have the problem of chain tension induced actuation, they also compromise the shock absorbing capabilities of the frame design and provide inferior performance. For example, some frames suspend the bicycle seat only, rendering the shock absorbing capabilities ineffective whenever the rider stands.
There is therefore a need for a suspension system which transforms substantially all of the driving force into forward motive force without substantial loss in the level of suspension and/or compromising the ideal rider position.